1. Field of the Invention
This invention relates to light emitters and, more particularly, to light emitters where the wavelength of at least some of the emitted light is converted to another wavelength by a conversion material region.
2. Description of the Related Art
Light emitting diodes (LEDs) are an important class of solid-state devices that convert electric energy to light energy and generally comprise an active layer of semiconductor material sandwiched between two oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. Light is typically emitted omnidirectionally from the active layer and from the surfaces of the LED.
One disadvantage of conventional LEDs used for lighting applications is that they cannot generate white light from their active layers. One way to cause a single blue emitting LED to emit white light is to surround it with a yellow phosphor, polymer or dye. [See Nichia Corp. white LED, Part No. NSPW300BS, NSPW312BS, etc.; See also U.S. Pat. No. 5,959,316 to Hayden, “Multiple Encapsulation of Phosphor-LED Devices”]. The surrounding material “downconverts” the wavelength of at least some of the light, changing its color. For example, if a blue emitting LED is surrounded by a yellow phosphor, then some of the blue light passes through the phosphor without being changed while the remaining light is downconverted to yellow. Hence, the LED emits both blue and yellow light, which combines to form a white light.
One conventional method for manufacturing an LED surrounded by a conversion material region is to mount the LED in a cup shaped submount with the necessary electrical connections to apply a bias to the LED. A syringe mechanism is filled with an optically clear and curable material (e.g. epoxy, silicone, sol gel, etc.) with the conversion material mixed in the material, where the conversion material typically includes phosphor particles. The syringe mixture is then injected into the submount, covering the LED and partially filling the submount. When the clear material is first injected into the cup, the conversion particles are generally uniformly mixed/suspended throughout the material. The clear material is then cured to form the conversion material region and the entire assembly is encased in a clear epoxy.
One disadvantage of this manufacturing method is that under certain circumstances the conversion particles can be non-uniformly distributed in the cured state. After the clear material mixture is injected into a cup, there can be a time delay before it is cured. During this delay, the conversion particles can settle toward the base of the cup and over the LED such that there are different concentrations of particles throughout the conversion material region. This settling problem can be compounded in clear materials that become less viscous during the curing process, which allows the conversion particles to settle more quickly. The settled conversion material region can result in light from the emitter appearing as different colors when viewed from different angles because the emitted light encounters different amounts of conversion material.
Another disadvantage of this method is that the injection of the optically clear material from a syringe can introduce variations in the concentration of conversion particles from emitter to emitter, which can reduce the consistent reproducibility of the emitters. The conversion particles can settle in the syringe such that emitters injected with the clear material mixture when the syringe is full can have a greater concentration of conversion particles than emitters formed later. The amount of clear material injected from the syringe into the cup can also be difficult to control and different amounts of clear material mixture can be deposited in different emitters. This can also result in different amounts of conversion particles in different emitters. The end surface shape of the cured material can also vary such that light from different LEDs pass through different amounts of clear material and particles. These problems reduce the ability to manufacture emitters with consistent light emission characteristics.
Another disadvantage of the conventional emitter manufacturing method is the waste of material when the emitter does not meet the necessary emission standards. There is no practical method for separating the two so the entire emitter must be discarded if the emitter or the conversion material region is defective. Hence, if the LED is good but the conversion material region is defective, then both will be unusable. Discarding the entire emitter results in the excessive wasting of otherwise good LEDs, which can add to the overall cost of manufacturing.